In the past, automotive manufacturers have found success with a number of different types of refrigerant compressors for use in air conditioning systems. These designs include arrangements wherein the cylinders in the cylinder block of the housing are arranged in an axial array, as well as arrangements wherein the cylinders extend in a radial direction. One of the most popular axial-type compressor designs includes multiple cylinders with double-acting pistons operating in cylinder blocks at opposite ends of the compressor housing. In this type of compressor, the cylinders are equally angularly spaced about and equally radially spaced from the axis of a central drive shaft. The double-acting piston is mounted for reciprocal sliding motion in each set of the opposed cylinders. The pistons are actuated in sequence by a drive plate, sometimes referred to as a swash plate. During operation of the compressor, rotation of the drive shaft imparts a continuous wave-type reciprocating motion to the drive or swash plate. This causes the drive plate to move in a nutating path around the drive shaft, that in turn serves to impart the required linear reciprocating motion to the pistons. In effect, there is no lost motion, since each stroke of the piston results in compression of the refrigerant gas in one or the other of the opposed cylinders.
A thorough description of the operation of this type of compressor is disclosed in U.S. Pat. No. 4,351,227 to Copp, Jr. et al. (assigned to the assignee of the present invention), issued Nov. 23, 1982. As described in this prior patent, the intake of refrigerant gas into the cylinders and discharge therefrom is controlled by unidirectional reed-type valves located on the valve plates at the ends of each cylinder block. Annular intake and discharge chambers are provided in the compressor heads at each end of the compressor.
As the drive shaft rotates the drive plate so as to reciprocate the double piston back and forth in the opposed cylinders, it will be realized that substantial rotary, as well as axial force must be accommodated. To do this, it has been standard practice to provide needle bearing assemblies to support the drive shaft for rotation at the front and the rear of the compressor housing and to provide standard thrust bearing assemblies positioned on both sides between the central body of the drive plate and the stationary hub portion of the opposed cylinder blocks that form the compressor housing.
As will be realized, it is necessary to assemble the compressor so that the interacting parts are very tightly held together, especially in the axial direction. As the pistons move on their compression stroke (in both directions), substantial axial forces are generated and transferred back to the drive plate. If the proper fit has not been obtained, the compressor is noisy, and any slight looseness will eventually cause inordinate wear in the thrust bearing assemblies, as well as in the other moving elements or components. In order to best resolve this problem, engineers in the past have simply required in the specifications for the manufacturing operation to hand-pick the races of the thrust bearing assemblies to accommodate the built-in tolerance. While the usual tolerance is only plus or minus 0.010 inch, this pre-selection requirement is slow and tedious and, above all, is subject to imperfection. Oftentimes, the exact match of bearing races is not available and, in that case, the manufacturing personnel simply selects the closest available combination, even though it is not a perfect match. Also, inaccuracies in measurement of the tolerance between the hub portion of the cylinder blocks and the body of the drive plate leaves much to be desired. It is very difficult to obtain an accurate measurement since many factors are present that can throw the measurement off. For example, built into the tolerance is not only the factor of the clearance between the internal rim of the hub portions of the housing and the annular shoulder on the body of the drive plate, but also the factor of the match of the outer annular machined joint of the opposed cylinder blocks. Also, in the past, the races of the thrust bearing assemblies have been fabricated of hardened steel so that wear per se has not been a problem. However, because of the relative rigid nature of the interface, and because of the substantial rotary and axial force components that are generated during operation, the parts in many circumstances, especially in hot, humid environments, are subjected to substantial intermittent peak loading as the compression forces are transmitted through the moving components back to the drive shaft. This condition can accelerate wear, and eventually sloppiness in the thrust bearing assemblies, and even fatigue in the metal, eventually leading to premature failure.
Despite substantial design changes, especially during the past several years, an additional factor contributing to noisiness and premature wear in axial compressors must be accommodated. That is, under certain operating conditions, even the best designed compressor may suffer from a condition known as "slugging". This condition occurs when lubricating liquid enters the cylinder bore and, as the piston begins its discharge stroke, it is forced to compress this liquid, as well as the refrigerant gas. Since the liquid is substantially incompressible, the discharge stroke of the piston is inhibited. Inevitably, the compressor components are subjected to higher loads and stress since the trapped liquid slugs cause simulated shock or impact loading, especially as the piston nears the end of its stroke. This action not only adds to the possibility of premature wear and failure due to the repeated excess force and torque loading, but greatly increases the noise of the compressor during operation. Accordingly, a need clearly exists for a design improvement in multiple cylinder axial compressors to reduce the adverse effects of these operating conditions, as well as to improve and to simplify the manufacturing process.